volume_mute

TDD and Legacy Systems

publish date: 2026/06/19 10:09:34.120071 UTC

volume_mute

TDD is difficult to apply when working with legacy systems. What is the primary reason for this difficulty?

TDD frameworks do not support connecting to legacy databases

Legacy systems are always written in languages that do not support automated testing

It is hard to write unit tests for legacy code because the code was not originally designed with testability in mind, often having tight coupling and no clear interfaces

Legacy systems are too large for TDD to be applied efficiently

Correct Answer

It is hard to write unit tests for legacy code because the code was not originally designed with testability in mind, often having tight coupling and no clear interfaces

Explanation

TDD is difficult with legacy systems because legacy code was written without testing in mind. It typically has tight coupling between components, few clear interfaces, and no separation of concerns - making it hard to isolate units for testing. Refactoring legacy code to make it testable is a significant effort in itself.

Reference

Software Engineering, Ian Sommerville, 10th edition

Quizzes you can take where this question appears

Test-Driven Development - Limitations and Context